1. Field of the Invention
This invention is related to the use of a charge-coupled-device (CCD) camera for the detection and differentiation of desired objects from undesired backgrounds which may be complex. The invention is particularly useful for detecting crops from undesired vegetation.
2. Description of the Prior Art
The difficulty of utilizing infrared (IR) imaging devices for the detection of cold targets is recognized in U.S. Pat. No. 5,132,802 which issued Jul. 21, 1992 to Osthues et al., entitled "High Contrast Image Apparatus Employing Optical Filters to Cause Each Image Pick-Up Element to have its Maximum Sensitivity in a Different Spectral Range." The apparatus of the Osthues et al. patent is described in the embodiment which is directed to the detection of military objects which may be surrounded by natural vegetation and may be camouflaged. The system of the Osthues et al. patent utilizes two image pick-ups which are selected to have two different wavelength ranges. In this patent the reflection from the vegetation is considered to be clutter. One of the wavelength ranges is selected so that the reflection characteristics of the object and of the surrounding natural vegetation are as similar as possible. The second wavelength is selected so that there is the greatest possible difference between the reflection behavior of the object to be detected and that of the vegetation.
Two separate CCD cameras are utilized in the Osthues et al. patent so that each camera receives and operates on one of the two wavelengths. The first wavelength, which is selected so that the characteristics of the object and the natural vegetation are as similar as possible, lies in the near infrared (NIR) range. This wavelength is equal to, or greater than, the range of 0.7 .mu.m to 2 .mu.m. The second wavelength, which is utilized to distinguish the background of the object, lies between 0.2 .mu.m and 0.6 .mu.m.
The Osthues et al. patent shows two separate embodiments for implementation of the optical device of this patent application. One of the implementations shows the use of two separate lens and filter combinations, each one of which is used to convey its image to a separate CCD camera. The second embodiment utilizes a single lens and two CCD cameras which are placed so that one of the camera devices is positioned horizontally and the other is positioned vertically or at right angles. The light from the lens is then directed onto an optical beam-splitter. The beam-splitter then splits the beam and directs a portion of it onto a filter providing the first wavelength to one of the cameras and a different filter for providing the second wavelength to the other camera.
It is well known that the ratio of the reflection of light from the near-infrared (NIR) and the red (R) portions of the spectrum is a reliable detector of living plants against various backgrounds of soil and crop residue. For example, this information is conveyed in the article entitled "Visible and Near-Infrared Plant, Soil, and Crop Residue Reflectivity of Weed Sensor Design" by B. Nitsch et al. in the ASAE Proceedings of the 1991 International Summer Meeting at Albuquerque, N. Mex., on Jun. 23-26, 1991, Paper No. 913006. This ratio of NIR/R signal strength is called in this paper the "vegetative index." An alternative ratio which may be used is the ratio of NIR-R/NIR+R, which is termed the normalized difference ratio, is described in this same article. The prior art indicates that the ratio of NIR/R may be utilized where the NIR is obtained from a portion of the spectrum from approximately 750 nm to 850 nm and the red is obtained from a region around 600 nm to 650 nm to differentiate plant growth from background soil and crop residue. Bandpass filters may be placed in front of photo-detectors to obtain the signals from the various spectral regions. This works well when the detection of living plants is the only requirement. However, when weed species must also be detected, identified and differentiated from the living crops, imaging techniques must be used. Two black and white CCD cameras with two separate filters could be used, but this can be a costly solution. In addition to the additional cost, this creates an alignment problem, especially on close objects since parallax errors will change the image alignment as a function of the distance of the object.
Another article in the Transactions of the ASAE (American Society of Agricultural Engineers) is entitled "The Use of Local Spectral Properties of Leaves as an Aid for Identifying Weed Seedlings and Digital Images" by E. Franz, et al. It appeared in Volume 34(2), March-April 1991. This article also discusses the use of NIR reflectance for plant discrimination and the use of a black and white CCD camera and photographic filters to extract spectral data from vegetative and non-vegetative regions.
In order to be commercially feasible for agricultural applications, the utilization of expensive cameras and optical systems must be minimized. In addition, a system which utilizes more than one CCD camera tends to be bulky and unmanageable. In the preferred embodiment of the invention, it is described for agricultural applications in which the sensor is intended to be mounted on a slow-moving agricultural vehicle.